Recent developments in wireless communications technologies have allowed expansion of service offerings from the original voice telephone service model to include a number of services supporting packet data communications. As customers become increasingly familiar with data services offered through landline networks, they are increasingly demanding comparable Quality of Service (QoS) data communications in the wireless domain, for example to maintain service while mobile subscribers roam freely or to provide remote service in locations where wireless loops are preferable to landline subscriber loops. A number of technologies support packet data communications in the wireless domain.
Under the currently proposed W-CDMA technical specification, there is only one type of dedicated transport channel, the Dedicated Channel (DCH), which can be either a downlink or an uplink transport channel. There are six types of common transport channels:
1. The Broadcast Channel (BCH)—downlink;
2. The Forward Access Channel (FACH)—downlink;
3. The Paging Channel (PCH)—downlink;
4. The Random Access Channel (RACH)—uplink;
5. The Common Packet Channel (CPCH)—uplink; and
6. The Downlink Shared Channel (DSCH)—shared downlink, associated with one or several downlink DCH.
With these transport channels, there are two states in the connected mode that can potentially be used to transfer packet data over the W-CDMA air interface: the Cell-FACH state and the Cell-DCH state.
In the Cell-FACH state, there are two sub-states: the RACH/FACH sub-state and the CPCH/FACH sub-state. A mobile station in the CPCH/FACH sub-state is prepared to send packets via the CPCH while tuned in to the FACH for downlink messages. In the Cell-FACH state, the Radio Network Controller (RNC) can allocate RACH or CPCH resources for uplink transmission. CPCH and RACH may be assigned by the RNC as default channels in the uplink without using uplink resources until they are needed for transmission of uplink data. RACH is able to transmit very small Packet Data Units (PDUs) effectively. RACH capacity is limited to 9 bytes at cell edge or to 75 bytes when the mobile station is close to the base station. Sequential RACH transmissions may be used to transport more PDUs than a single RACH may carry, however, the RACH access procedure must be executed for each RACH access and the subsequent delay is significant. The RNC sets a threshold measurement of traffic volume in the mobile station, essentially instructing the mobile station to send a measurement report to the RNC when, for example, the traffic volume in the mobile station uplink buffer exceeds the capacity of two RACH transmissions. That would be the load at which it would make sense to utilize a higher capacity channel to transmit the buffered uplink data. If the measurement report is triggered, the RNC may assign CPCH resources to empty the uplink buffer or can switch the mobile station to Cell-DCH state.
CPCH may be assigned instead of RACH, to provide higher capacity uplink transport. A single CPCH access may transport up to 576×16 bytes of data at the cell edge (64 frames at SF 16) or up to 36,864 bytes when the mobile station is near the base station (64 frames at Spreading Factor 4). When CPCH resources are assigned to a mobile station, the RNC sets a threshold measurement of traffic volume in the mobile station, essentially instructing the mobile station to send a measurement report to the RNC when traffic volume in the mobile station uplink buffer exceeds the capacity of five to ten CPCH transmissions. Consecutive RACH or CPCH accesses may be used until the uplink buffers are emptied.
In the Cell-DCH state, there are the DCH/DCH sub-state and the DCH/DCH+DSCH sub-state. That means the mobile station sends packet data via the DCH uplink and is tuned to receive data downlink via either the DCH or the DCH+DSCH. The DSCH is a code-sharing mechanism in the downlink direction and is more desirable when data traffic is bursty. The DCH is more suitable for streaming traffic and is not a resource efficient means of transmitting bursty uplink data. In the uplink, DCH is different in that dedicated resources in the uplink must be allocated by the RNC without complete knowledge about the amount of data to be transmitted in the uplink. For this reason an inactivity timer is used in DCH to determine if the uplink buffer at a mobile station is emptied. The RNC will measure the time period in the uplink during which there is no uplink data transmission. When this period exceeds the inactivity timer setting, the RNC will reconfigure the mobile station to Cell-FACH. In the downlink, the Radio Network Controller (RNC) can allocate either DCH or DCH+DSCH resources for packet data transmission. Similarly, the RNC does not have complete knowledge of future packet arrivals and uses instead inactivity timers to measure the time period in the downlink during which there is no data transmission. When this period exceeds the inactivity timer setting, the RNC will reconfigure the mobile station to Cell-FACH. These inactivity timers in CELL-DCH lead to substantial overhead and inefficiencies when the data traffic is bursty, thus reducing capacity.
For certain types of packet data applications (e.g. interactive service), ideally, one would like to use a Cell-FACH (e.g. CPCH/FACH sub-state) for uplink traffic and switch to a Cell-DCH state (e.g. DCH/DCH+DSCH sub-state) for downlink traffic. The reason is that there are certain deficiencies with both states. In Cell-FACH state, FACH downlink does not have closed loop power control and has only limited capability to handle large packets, whereas in the Cell-DCH state, as in any circuit-switched packet channel, there is a lot of wastage of limited resources. However, a problem with the proposed frequent switching is that a mobile station while residing in the Cell-DCH state cannot be de-allocated immediately after transmission of packet data due to the inactivity timer.
Also, when a group of packets arrive from afar, as in the case of a backbone network, there will often be time-gaps between these packets. When the RNC assigns channel resources immediately after the arrival of the first packet and does not release such resources until the last packet of the train arrives, the channel hold-up time will increase, thus creating inefficiencies.